1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to golf balls including a center, a mid layer, a cover and dimples.
2. Description of the Related Art
The greatest interest to golf players concerning golf balls is flight distance. In particular, golf players place importance on a flight distance upon a shot with a driver. By using a golf ball which has a long flight distance upon a shot with a driver, golf players can hit a second shot at a point close to the green.
Golf players also place importance on flight stability. By using a golf ball which has less-variable flight distance and flight direction, golf players can let the golf ball to land on a target point.
Further, golf players place importance also on spin performance of golf balls. If a backspin rate is high, the run is short. By using a golf ball which has a high backspin rate, golf players can let the golf ball to stop at a target point. If a sidespin rate is high, the golf ball tends to curve. By using a golf ball which has a high sidespin rate, golf players can intentionally let the golf ball to curve. A golf ball with excellent spin performance has excellent controllability. In particular, high-level golf players place importance on controllability of a shot with a short iron.
Golf balls have a large number of dimples on the surface thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. By causing the turbulent flow separation, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulent flow separation promotes the displacement between the separating point on the upper side and the separating point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. The reduction of drag and the enhancement of lift force are referred to as a “dimple effect”. Excellent dimples disturb the air flow more efficiently. The dimple effect provides a long flight distance.
Generally, golf balls are formed with a mold having upper and lower mold halves each having a hemispherical cavity. Assuming that the upper mold half cavity is the northern hemisphere of a globe and the lower mold half cavity is the southern hemisphere of the globe, the upper mold half and the lower mold half are mated at the equator face (the plane including the equator). A large number of pimples are provided on the inner surface of the mold, and dimples are formed on the surface of the golf ball by means of the pimples. The dimples have a shape inverted from the shape of the pimples.
Since the molding material (e.g. synthetic resin) leaks out from the parting face of the upper mold half and the lower mold half, a flash is generated along the equator portion on the surface of the golf ball. The flash is generated along the parting line. The flash is ground and removed with a whetstone or the like. Removal of a flash generated inside the dimple is difficult. In order to facilitate the removal of the flash, any dimple is not formed on the equator. In other words, no pimple is provided on the parting face of the mold. A great circle path is formed on the seam of the golf ball obtained by this mold. The great circle path agrees with the equator. The great circle path does not intersect with any dimples. When this great circle path agrees with a part where the fastest circumferential speed of the backspin is attained (hereinafter, may be also referred to as a “fastest part”), a sufficient dimple effect can not be achieved. The dimple effect achieved when the great circle path agrees with the fastest part is inferior to the dimple effect achieved when the great circle path does not agree with the fastest part. The difference between these dimple effects deteriorates the aerodynamic symmetry of the golf ball. The great circle path further impairs the flight stability of the golf ball.
JP2002-159598 (US2002/94886) discloses a mold provided with a parting face having a horizontal plane and an inclined plane. In this mold, pimples can be arranged on the equator except at the parting face. This mold provides a golf ball having a non-smooth seam. This golf ball does not have a great circle path.
JP10-99469 (U.S. Pat. No. 5,947,844) discloses a mold having pins placed on the parting face. The pins form dimples on the golf ball. By this mold, a golf ball without a great circle path is obtained.
JP11-137727 (U.S. Pat. No. 6,123,534) discloses a mold provided with a parting face having bulges. The bulges form dimples on the golf ball. By this mold, a golf ball without a great circle path is obtained.
JP2005-224514 (US2005/181891) discloses a golf ball with a cover including thermoplastic polyurethane. The thermoplastic polyurethane contributes to the controllability of the golf ball.
JP11-178949 (U.S. Pat. No. 6,123,628 and U.S. Pat. No. 6,582,325) and JP2002-336378 (US2003/64831) each disclose a golf ball with a cover which is formed of a resin composition including polyurethane and an isocyanate compound. This resin composition contributes to the controllability of the golf ball. In addition, this isocyanate compound enhances the scuff resistance of the cover.
Upon a shot with a driver, the lower the spin rate is, the longer the flight distance tends to be. On the other hand, as described above, in light of controllability with a short iron, a higher spin rate is preferred. Recently, golf players' requirements for golf balls have been escalated more than ever. Golf players desire golf balls to have a long flight distance, less-variable flight direction and flight distance, and excellent controllability. An objective of the present invention is to provide a golf ball having a long flight distance, excellent flight stability, and excellent controllability.